Batteries store energy in chemical form. A rechargeable battery is a type of battery capable of transmuting electrical charge by storing it in the form of a reversible chemical reaction. When the battery is subsequently placed across a load, this reversible reaction reverses from the direction in the storage mode, thereby producing electrical energy for use by the load.
There are many popular types of rechargeable batteries. Perhaps the most popular are the nickel cadmium and lead acid types. These batteries generally operate over a usual range of ambient temperatures, and store a relatively small amount of charge.
Other types of battery, which are presently becoming more popular due to relatively high amount of energy storage, are generically labeled as "high temperature batteries". Examples of these kinds of batteries are electrochemical storage cells of the alkali metal and chalcogen type, sodium sulfur type, and lithium or lithium fluoride type. Operation of a high temperature battery requires it to be stored in an ambient environment with a temperature of between 300.degree. C. and 500.degree. C. Thus, although these high temperature batteries can store increased amounts of charge, they must be used under difficult operating conditions (very high temperatures). To compound this problem, typically the reliability of these batteries is reduced by any heat cycling of the batteries between ambient temperature and their proper operating temperature. Thus, when using these batteries, continuous reliable operation is highly advantageous. The importance of trouble free operation is made doubly evident when it is considered that even if servicing of these batteries is desired, the process requires a significant amount of cooling time before the parts of the batteries would be cool enough to be handled by service personnel. During this cooling time, the batteries will necessarily be out of service.
Single battery cells are generally available in the range of approximately a half a volt to five volts, with the specific voltage of the battery cell depending on properties of the chemical reaction which is occurring within the battery cell. Thus, to obtain a battery which has a higher voltage than this relatively low cell voltage, typically a plurality of cells are placed in series, to thereby add the respective cell voltages to obtain a resultant higher voltage. Sodium sulfur batteries, for example, have a cell voltage of approximately two volts. Thus, should a battery voltage of 48 volts be required, 24 of the cells would be required to be connected in series. However, this connection in series presents some problems with respect to operational reliability and to maximum battery efficiency.
One such problem is due to the fact that when a battery cell fails it will typically fail into the open circuit state. This failure into open circuit state would effectively destroy the utilitarianism of the entire battery. Thus, in the example given above of a 48-volt battery, a single cell of the 24 cells in the battery malfunctioning into an open circuit state would necessitate the replacement or repair of the entire battery. As such, it would be advantageous in the art to have a device which obviates this problem.
An early attempt at such a device is described in U.S. Pat. No. 2,624,033. This patent teaches placing individual diodes in parallel across each series connected cell. These diodes are placed so that normally a charged cell would reverse bias these diodes. However, when a cell either open circuits or is discharged close enough to zero volts, the related diode shunt across the particular cell will be forward biased. Thereby the particular cell will be effectively shorted out. The patented system has the disadvantage that in order for the diode to shunt across the particular cell, the diode must be forward biased and operating correctly. There is no permanent state change in the diode, and thus a failure of the diode would cause the battery to malfunction.
Another proposed solution to the problem is suggested in the disclosure of U.S. Pat. No. 3,102,222. This patent teaches a device which is specialized to high temperature catalytic battery cells, whereby by sensing the temperature of a particular battery cell, the condition of that particular cell can be approximated according to a predetermined algorithm. A switch 1 is normally closed and connected in series between the battery and the charging unit. The switch 1 is arranged to open when the temperature of the catalyst used in the battery reaches a predetermined value. The patent does not teach a method of shunting across individual cells in response to cell failure. Furthermore, this technique would only be applicable to high temperature battery cells.
A further proposed solution to the problem is taught in U.S. Pat. No. 4,303,877, the disclosure of which is expressly incorporated herein by reference. This patent teaches a plurality of battery cells of the electrochemical storage type in series. Shunted across each such cell is a temperature sensitive switch and a diode in series with heating device. In one preferred embodiment, when a cell fails into the open circuit state, the diode is forward biased thereby energizing the heating element. This heating element then heats the temperature sensitive switch, which permanently changes position--similar to a fusible link. This temperature sensitive switch thus permanently changes position in response to a cell of the battery failing. The failed cell is thereby effectively shorted across. While the general technique used is extremely effective, a disadvantage exists in the relative complexity and impracticality of the many components being used within a high temperature battery of 300.degree.-500.degree. C. The present invention overcomes all these problems by a single component performing all these functions as described herein. This operation of the present invention is extremely advantageous in high temperature battery cells, such as sodium sulfur. However, although the present invention finds a great usefulness in high temperature batteries such as sodium sulfur, it is not intended to be limited to these kinds of batteries and would find many applications in low temperature batteries such as nickel cadmium and lead acid as well. Since the requirements of a high temperature battery make maintenance of the battery difficult, these devices are particularly cost justified in these high temperature batteries.
Thus, it is an object of the present invention to overcome the problems stated above, by use of a specially constructed silicon diode placed in series across each battery cell. The preferred embodiment of the invention teaches a silicon diode looping element which, upon failure mode, permanently changes its conductive state to a short circuit, thus effectively shorting out the malfunctioning cell. Thus, the diode operates as a fusible link, permanently bypassing the failed cell, and enabling the battery to continue operating although at a slightly lower battery voltage .